Optical communications cables consist essentially of a plurality of randomly placed optical communication fibers, typically in ribbon form, contained within a polymer jacket. Of course, other types of communications cables can have single optical fibers, bundled optical fibers, or tight buffered optical fibers. The fibers are typically tight or loose buffered within a polymer tube contained within a portion of the jacket. One or more flexible reinforcement members and stiff strength members may also be contained within a portion of the polymer jacket outside of the central tube or buffer tubes to provide support and prevent kinking of the communication fibers. These reinforcement members are applied by being wrapped helically around the inner core prior to outer jacketing (as in optical loose tube cable types) or are directly extruded into the jacket matrix (as in twisted copper telephone cable).
The flexible reinforcements for cables are made in a wide variety of methods. Typically, these reinforcements are manufactured by first applying a binder and sizing containing a film former and oils or coupling agents to a glass strand and then applying a relatively heavy layer of a water-based, high molecular weight polymer latex or wax. The coated strands may then be introduced to the communications cables by known methods.
One problem with presently available reinforcements is that they are relatively expensive to manufacture. For example, a relatively heavy layer of high molecular weight polymer latex or wax must be applied to the fibers in order to impart the mechanical properties necessary for optical and copper telecommunications cables. Also, these high molecular weight polymers have extremely high melt viscosities. Further, air can be trapped within the interstices of the fibers themselves after the introduction of the high molecular weight polymers, which can lead to premature degradation of the fibers and strand deficiencies. Also, because water-based high molecular weight coatings are typically used, a high-energy water-removal step is required before the fiber reinforcements can be introduced into the cabling. These water-based coatings, typically in emulsion form, are expensive as well.
These coatings may impart many important properties to the flexible reinforcements both during manufacture and after introduction to the cable. For example, these coatings prevent abrasion of the glass fibers during the combination with the reinforcement and during deployment. Also, these coatings prevent adhesion of the reinforcing fibers to the polymer jacket. These coatings may also impart adhesion if desired to the polymer jacket, for example, as is the case with polyvinyl chloride (PVC) jacketed communications cables. Additionally, these coatings can be super absorbent and can thus prevent water seepage from damaging the optical fibers.
Deterioration caused by the invasion of moisture beneath the exposed surfaces of articles used in outdoor environments is a well-known problem. This deterioration includes oxidative deterioration caused by reaction of water with the surfaces of reinforcing fibers used in these articles, as well as water-induced corrosion. In marine environments, for example, the problems associated with water logging are particularly compounded by the salinity of the environment. The presence of salt in such aqueous environments hastens the oxidative decomposition. In non-saline environments, for example in environments having high atmospheric humidity, water-resistant coatings are necessary to protect the structures and equipment surfaces from moisture-induced decomposition.
Articles affected by the deterioration described above include items having a surface exposed to high moisture or humidity. Examples of such articles include reinforced rods and cables, such as fiber optic or telecommunications cables. These telecommunications cables are often used in situations where they are buried underground or submerged in water over long periods. As such, protection from water damage is critical to the structural integrity of these cables and to the success of the functions they are intended to perform. A telecommunications cable, for example, may include a core comprising a glass rod that acts as a stiffening or reinforcing member. This rod contributes to the rigidity of the cable. When water penetrates to contact the core element of the cable, corrosion or chemical deterioration of the cable infrastructure may result.
U.S. Pat. No. 5,925,461 teaches a hot melt coating adhesive having a 20–50 parts by weight dispersion of a water-swellable particulate material to 100 parts by weight of hot melt. The above patent further teaches the process of using a heated bath with the above formulation, submerged saturator bars, stripper die, shaping rollers and winder.
The process described in the above patent has difficulties. The recommended hot melt material is an ethylene vinyl acetate (EVA) polymer. The superabsorbent particulate is most likely a sodium or potassium polyacrylate salt, also referred to as a superabsorbent polymer (SAP). Examples of such polymers include, but are not limited to, ethylene vinyl acetate (EVA) polymers, block copolymers of polybutylene terepthalate and long chain polyether glycols, thermoplastic elastomers, olefins or urethanes, polypropylene, polyethylene, polyurethane or low molecular weight mineral wax. Polyacrylamides may also be utilized.
It is well known that polymers such as EVA are high molecular weight and subsequently display very high melt viscosities, even at very elevated temperatures.
Typical sizes, binders and water-based impregnants have viscosities in the 20–200 cp range. These lower viscosity fluids easily penetrate the glass fiber bundle, coating individual glass filaments, and results in both well protected fiber-fiber abrasion, as well as subsequent high measured tensile strength, due to the good stress translation afforded by the complete wet-through of the matrix coating.
Conversely, attempting to impregnate a bundle of glass filaments with a highly viscous fluid or melt, such as that taught in U.S. Pat. No. 5,925,461, would quickly result in extremely high shear forces between the glass filaments and the submerged bars in the bath. These forces could quickly exceed the individual tensile strength of the fibers, breaking out individual filaments and eventually breaking out the strand. This can be minimized if the process moves very slowly, probably much less than 10–30 meters/min. Even then, the very viscous mixture would have great difficulty efficiently penetrating the interstices between glass filaments unless other process equipment is installed, such as heated applicator drums or rollers. Additionally, adding extra equipment in general complicates a process line and makes it more prone to disruptions.
The present invention solves the above problems in fabricating water swellable, semi-flexible cable reinforcements of small diameter by using blends of hot melt coatings in various coating sequences. In another embodiment, a previously coated superabsorbent strand is coated with a hot melt coating.